Amid concerns over global warming and forecasts of depletion of drinkable water and non renewable energy sources, solar energy has become a viable option. By generating solar energy by means of a solar thermal power plant, energy may be produced in an environmental friendly manner, generation of energy is potentially limitless, and energy may be produced in a substantially simple way.
A known type of a solar thermal power plant utilizes a “radiation concentrator collector” which concentrates solar radiation by focusing said radiation into a smaller area, for instance by using mirrored surfaces. In such a known type of solar thermal power plant, a reflector, which is typically parabolic, receives and reflects (focuses) incoming solar radiation onto a radiation absorber, which is formed as a tube. The radiation absorber tube may conventionally be surrounded by a treated glass tube to limit the loss of heat. The space between the radiation absorber tube and the glass tube may be a vacuum space.
The radiation absorber tube may be made of metal with a coating having a high absorption coefficient for solar radiation to maximize the energy transfer from the reflector. A thermal fluid, constituting a heat transfer medium, which is typically a liquid such as oil, flows within the radiation absorber tube.
The thermal energy transported by the thermal fluid is used to generate steam which in turn is expanded into a turbine, in order to generate electricity in a conventional way, by coupling the axle of the turbine to a generator. Then, the steam is condensed and recycled back to a heat exchanger system. The cycle to transfer steam into electricity may be more efficient at high steam temperatures. However, some heat transfer media may dissociate or become less effective in transferring heat if said heat transfer media operate at very high temperatures. Due to this restricting behaviour, conventional plants generating solar thermal energy are typically limited to operate at about 400° C. At such temperature the overall efficiency of the cycle to transfer steam into electricity may be limited to less than 32%.
Of course such a known solar thermal power plant also has an operational limit due to the occasional absence of solar radiation.
In order to extend the operation time of such solar plants, additionally a known thermal storage system based on a molten salts mixture may be used. Such a known system may comprise two thermal storage tanks, a cold fluid storage tank and a hot fluid storage tank. During normal operation, thus when solar radiation to heat the heat transfer medium is present, a fraction of the stream of the heat transfer medium, such as oil, may be used to heat the molten salts mixture that is supplied from the cold storage tank to the hot storage. In case no solar radiation is available, the molten salts mixture flow is reversed. Then, the hot molten salts mixture is used to heat the oil and is supplied from the hot storage tank to the cold storage tank. A drawback of using such system is that a substantial part of the capital investment of conventional solar thermal energy generating plants is absorbed by the molten salt storage system, despite the fact that such a system enables to produce power according to the demand conditions which significantly improves the overall economics.
A further known solar thermal power plant may be based on using molten salts as heat transfer fluid (HTF). Such a solar thermal power plant always needs to operate at a temperature higher than the solidification temperature of the molten salts (around 240° C. for the Na/K nitrates mixture). Auxiliary heaters or boilers may additionally be used. However, the known solar thermal power plants may still have efficiency problems.
Therefore, it is an object of the present invention to provide an improved boiler system for a solar thermal power plant. More in particular an object of the invention is to provide a boiler system for a solar thermal power plant that operates on a molten salts mixture and which allows efficient generating of electricity, preferably under any weather condition and at a low sun level, such as at night.